1. Field of the Invention
The invention relates to a cryptographic receiver, and more particularly to a cryptographic receiver for a communications system in which a cryptographic key-code is derived by two parties as a result of an exchange of information between the two parties following measurement of individual photons.
2. Discussion of Prior Art
Cryptographic receivers incorporated within cryptographic communications systems have been described previously, for instance in International Application No. PCT/GB91/01352 with a Publication No. of WO92/04785, and by A. K. Ekert et al, in Physical Review Letters, vol 69, number 9, August 1992, pages 1293 to 1295. The communications systems described therein comprise two cryptographic receivers separated by some distance and a source of correlated photon pairs. The receivers each include a filter whose transmission characteristics are modulated in response to a respective control signal. Exchange of information about the control signals between the two receivers enables each one to derive a common key-code, available to the receivers only. This key-code enables further messages to be encrypted and transmitted using insecure communications links. These systems have the disadvantage that high speed modulators are required which have associated high transmission losses.
A cryptographic communications system based on a photon polarization measuring receiver has been described by A. Muller et al, in European Physics Letters, vol 23, 1993 pages 383 to 388. The receiver described therein comprises an electro-optic switch which switches photons between two channels, one of which contains a 45.degree. rotator. The channels recombine and photon polarizations are separated by a polarization beam splitter before being detected.